It is generally known that telephone service is sometimes abused by persons making fraudulent calls, that is, calls that cannot be billed properly. One method of placing a fraudulent call involves the use of the so-called "blue box" device. The blue box device permits a calling party to simulate in-band supervisory signals and multifrequency tones which are normally generated by the switching equipment to direct calls through the toll network. The blue box user generally places an inexpensive toll call to a nearby destination or a call to a toll free number. At the appropriate time, when the message billing equipment is waiting for an answer signal from the destination office, the caller transmits a disconnect signal to the destination toll office. In response to the signal, the destination office abandons the call in progress. When the blue box user removes the signal, it is interpreted at the destination toll office as a new request for service. The destination office attaches a register-sender to the connection and returns a "proceed-to-send" or "delay dial" signal towards the calling party. The blue box caller can now key in a new number and direct this second call anywhere in the toll network. The message billing equipment is unaware of the second call and charges the calling customer on the basis of the inexpensive or free call to the first number.
Attempts have been made to detect calls from a blue box user. In one known arrangement, as disclosed in U.S. Pat. No. 4,001,513 to D. C. Naylor of Jan. 4, 1977, the second "proceed-to-send" signal is recognized at the time when answer supervision is normally expected from the called customer. In this arrangement, a multifrequency receiver is coupled to the connection to record the tones transmitted by the calling customer's blue box.
While the Naylor arrangement is wholly suitable for its intended purpose, there are no provisions set forth therein for blocking the fraudulent call from completion.
Another known arrangement of fraud detection which blocks the establishment of the fraudulent call is disclosed in U.S. Pat. No. 4,002,848 to R. P. Stein of Jan. 11, 1977. In the Stein disclosure tone detectors are coupled to the connection at a point where in-band supervisory signals are not normally present. Thus, if a single frequency supervisory signal is present, the call is presumed to be a fraudulent call and the sleeve lead of the connection is opened to release the switch train.
While the Stein arrangement prevents the completion of the fraudulent call, it requires that all lines be scanned looking for the tones generated by the blue box. In addition, once having detected a fraudulent call, the connection is abruptly interrupted and the calling party may be made aware that his fraudulent call has been detected.
Accordingly, a need exists in the prior art for a more efficient arrangement for detecting and frustrating fraudulent toll calls while minimizing the risk of warning the fraud perpetrator.